1. Field of the Invention
The present invention relates to a telephone communication system, and can be applied to, for example, a telephone communication system including a telephone that can be connected to an IP network via a network connection apparatus.
2. Description of the Related Art
In recent years, a VoIP technology has integrated data and voice by translating the telephone signal into IP packets, thereby lowering network and communication costs, and thus has become widely used.
Moreover, the traditional public switched telephone network (PSTN) uses a band of 3.1 KHz or lower for communication, focusing on transmission of voice signals. That is, the terminal (narrowband telephone) side and the network side both treat signals in a band of higher than 3.1 KHz as unnecessary.
However, with the rapid progress of broadbandization in recent years, not only transmission equipment on the network side has become broadband-compliant, but also subscriber lines have been switched to broadband lines using ADSL, light or the like. Accordingly, end-to-end wideband voice communication has become possible, and thus wideband telephones are appearing.
Moreover, it is being researched and developed to enable narrowband telephones to perform quasi-wideband communication by adding a correction function between narrowband signals and wideband signals to gateway apparatuses.
Furthermore, because various telephones exist, the cases of using network connection apparatuses (gateway apparatuses) are increasing for wideband communication via a wideband network (wideband IP lines) and narrowband communication via a narrowband network (narrowband IP lines or the public switched telephone network). Also various gateway apparatuses are being proposed and developed corresponding to different networks where they are used.
As described above, there are various telephones, various gateway apparatuses, wideband networks, and narrowband networks, and thus there are many possible combinations of telephones, gateway apparatuses, and networks for a given telephone call.
However, in the case of a combination with which wideband communication is not possible over the entire end-to-end path, if communication is performed with narrowband signals over the entire end-to-end path, apparatuses with high transmission capability will be used wastefully.
Furthermore, VoIP-compliant gateway apparatuses and telephones, which have been widely used in recent years, with emphasis on being VoIP-compliant, have not realized high quality voice communication making use of the characteristics of broadbandized networks.
Yet further, it is unknown at the design stage of a telephone whether the telephone is connected to the public switched telephone network or an IP network and also it is unknown whether to be connected via a gateway apparatus to a network. Hence, even if the telephone is planned to have high quality and be wideband-compliant, it often occurs that sending and receiving signals is not appropriately performed when actually installed in a communication system and connected to, for example, a gateway apparatus.
Furthermore, it is being examined to correct the characteristics of a narrowband telephone by a gateway apparatus such that the narrowband telephone looks like a quasi-wideband telephone as described above, but it is not clear at what trigger they should be corrected, and hence there is a possibility that the correction will not bring out the highest performance.